1. Field of the Invention
This invention relates generally to a liquid storage tank, and more particularly to an apparatus and method for accurately determining the amount of liquid contained in the tank.
2. Description of the Prior Art
Liquid storage tanks are used to contain various substances in a wide range of applications. It is usually of interest, and often a necessity, to determine the level of liquid in a storage tank. In applications where knowledge of the precise quantity of liquid is critical, an indication of volumetric content is required. Applications involving the storage of dangerous substances, such as Benzene, typically require an enclosed tank completely sealed off from the outer environment. In these instances, measuring the level/volume of liquid in the tank must usually be done external to the tank. Additionally, some applications require an indication of the amount of liquid contained in the storage tank at a location remote from the tank.
Many sensing or monitoring approaches have been developed for determining liquid level. In some cases, rudimentary means are sufficient to determine liquid level, such as a sight glass connected to or incorporated into a side wall of the storage tank. In other cases where access to the side of the tank is impossible, inconvenient or impractical, a measuring rod may be inserted through an opening in the top of the tank to manually measure the liquid level. One end of the rod is initially placed in abutment against the bottom of the tank and the rod is then withdrawn. The level of liquid in the tank is determined by simply observing how much of the rod is wetted. In a more refined approach, a tube is mounted in the tank in a substantially vertical orientation. A float encircles the outer wall of the tube and is magnetically coupled to one end of a measurement rod. As the liquid level rises, the float carries the rod upwardly, whereby the level of liquid in the tank is indicated by the length of rod protruding from the upper end of the tube. Typically, the rod is graduated, having indicia imprinted on the outer surface of the rod which signify the amount of liquid contained in the tank.
Recently, even more refined and elaborate approaches have been developed to monitor liquid level. For instance, ultrasonic pulse-echo ranging systems have been used to determine the level of liquids and solids in tanks and other containers. However, a problem encountered when using such equipment is that the speed of sound varies markedly, both with temperature and according to the composition of the atmosphere between the ultrasonic transducer and the liquid level being measured. Furthermore, the high electrical voltages required to energize the transducer can pose significant problems in meeting safety requirements, especially where highly flammable vapors are contained in the tank.
Another method which has commonly been used to measure liquid level is dielectric variation sensing or capacitive liquid level sensing. This method is primarily used in conjunction with relatively non-conductive liquids. The non-conductive liquid plays the role of a dielectric material disposed between multiple electrodes which are used and are electrically connected as plates of a capacitor. The capacitance changes continuously as the vertical height of the liquid in the tank rises or falls. However, a problem with this approach is that it is often necessary to compensate for changes in the characteristics of the liquid being measured. Accordingly, because of the elaborate nature of the ultrasonic and dielectric methods of measuring liquid level, utilization of these methods is expensive and often requires a relatively high degree of maintenance.
As mentioned above, in certain applications involving the storage of dangerous substances, the interior of the storage tank must be sealed off from the outer environment. According to the nature of the liquid being monitored, which may be flammable, corrosive or have other undesirable properties, it may be necessary to isolate the liquid and/or its resulting vapors from the device used to measure the liquid level. Moreover, for measuring devices which require a relatively high degree of routine and periodic maintenance, it is often preferred, and sometimes absolutely required, that maintenance be conducted exterior to the storage tank. In many cases, it is both expensive and impractical to gain access to the interior of the storage tank, especially where the vapors given off by the liquid are noxious or poisonous.
In many liquid storage tank applications, mere knowledge of the level of liquid contained in the tank is insufficient since the volumetric content of liquid contained in the tank may not directly correspond to liquid level. Most methods currently used to determine the amount of liquid in a storage tank provide only a linear model of the "volume profile" (i.e., the internal dimensions of the storage tank). Although such a system is capable of indicating the general presence of liquid in the tank, such an indication fails to provide accurate information regarding the exact quantity of liquid contained in the tank, especially in tanks having a non-linear volume profile. In other words, these methods are incapable of accurately determining the amount of liquid contained in a storage tank where the quantity of liquid contained therein varies in a non-linear relationship relative to liquid level.
In order to compensate for tanks having a non-linear volume profile (tanks having contoured interior walls or irregular shapes), a computer chip is commonly employed to convert the liquid level to a numeric value corresponding to the volume of liquid contained in the tank. However, such methods requiring the use of a computer chip to convert a linear measurement to a volumetric unit of measurement are typically complex, and as a result are expensive and subject to increased maintenance costs. Alternative methods for determining the quantity of liquid in a storage tank are disclosed in U.S. Pat. No. 4,724,705 to Harris, U.S. Pat. No. 4,215,574 to Godeaux, and U.S. Pat. No. 4,192,187 to Kennedy et al. However, these alternative methods utilize devices which are also relatively complex, expensive, and difficult to maintain.
The present invention attempts to remedy the inconveniences and drawbacks of prior gauge devices, and provides an apparatus and method for accurately determining the amount of liquid contained in a storage tank having an interior defining a predetermined relationship between liquid level and volumetric content.